1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus for reading out image signals with pixels reduced in a horizontal direction, and a signal reading method for the same. The present invention is advantageously applicable to, e.g., a digital camera or an image input apparatus including an image pickup section having high pixel density.
2. Description of the Background Art
To implement image quality comparable with one available with a silver halide photo-sensitive type of film, there have been proposed various technologies for increasing the number of pixels of a digital camera that electrically shoots a scene. Japanese patent laid-open publication No. 136391/1998 discloses a solid-state image pickup apparatus constructed to optimize the spatial sampling of an image, to shift pixels with respect to each other in such a manner as to enhance efficient receipt of light, and to reduce moiré and other aliasing signals.
A digital camera of the type including an image pickup section provided with high pixel density is extensively used and directed toward high image quality. It is a common practice with this type of digital camera to effect, before the actual pickup of a still picture, AE/AF (Automatic Exposure/Automatic Focusing) operation and movie drive that causes a scene being picked up to appear on an LCD (Liquid Crystal Display). This, however, brings about a problem that the high pixel density increases the period of time necessary for signal charges resulting from the pickup to be read out and thereby lowers the frame rate. It is to be noted that the high pixel density refers to more than 1,000,000 pixels or so-called megapixels.
To increase the frame rate, signal charges generated in the image pickup section may be read out while being reduced, or thinned, in the vertical direction. Specifically, assume that drive frequency CLK for reading out all of 1,500,000 pixels (1,280×1,024) by progressive scanning is 12.2725 MHZ. Then, a single horizontal synchronizing period (1 H) and a single vertical synchronizing period (1V) are 1,560 CLK and 1,050 H, respectively. The frame rate is therefore 1/7.5 second. When the signal charges are reduced to one-half in the vertical direction, 1 H needs the same period of time while 1V is 525 H, resulting in a frame rate of 66.7 milliseconds, i.e., 1/15 second. Even when the signal charges are reduced to one-fourth in the vertical direction, 1V is 262.6 H, and therefore the frame rate is as long as 33.4 millisecond or 1/30 second.
Assume that 1,500,000 pixels are read out by progressive scanning and displayed by the movie drive in the conventional image size, i.e., 640×480. Then, the pixels are reduced to one-half in the horizontal and vertical directions under the above-described conditions. As a result, the number of pixels in the horizontal direction and the number of pixels (lines) in the vertical direction are as great as 640 and 525, respectively. Even the reduction to one-fourth implements only the reduction to one-half in the horizontal direction although reducing the number of pixels in the vertical direction to 262.5, i.e., improving the frame rate. However, because the number of pixels reduced in the vertical direction is short of 480, interpolation must be executed in the vertical direction in order to match the number of pixels to the desired number. On the other hand, in the horizontal direction, all of the 1,280 pixels are read out and then reduced to 640 pixels at the subsequent signal processing stage. It will therefore be seen that strict consideration is not given to the improvement in frame rate in reducing the pixels in the horizontal direction. This is apt to prevent the operator of the camera to miss an adequate actual pickup timing.